Clock signals are essential to the operation of modern integrated circuits, in which the operations are synchronized by clock signals, which often contains a repeated pattern of “1010.” To speed up the integrated circuits, the clock signals have to be speed up. Currently, the clock signals have the frequencies in giga hertz (GHz) ranges. It is expected that by year 2010, the clock signals will be in the range of about 15 to 20 GHz. In this range, the electrical signal begins to merge with optical signals.
With the increase in the frequencies of clock signals, more heat is generated by the clock signal circuits, for example, flip-flops. Significantly greater power consumption and more severe heat-dissipating problems are hence resulted. With the increased heating and increased frequencies, glitches and jitters also become more severe, hence preventing the further increase in the frequencies of clock signals. Various methods (U.S. Patent Publication Nos. 2005/0145783, 2005/0223683, and 2003/0094699) have been explored to introduce optical clock signals into semiconductor chips, and converting the optical clock signals in the semiconductor chips to electrical clock signals. In these methods, photon detectors are used to convert the optical clock signals to the electrical clock signals. 
The conventional methods, however, suffer from drawbacks. For example, if the optical signal is introduced into substrates from the front sides of the respective semiconductor chips, then the metallization layers, which already have high densities, will prevent efficient allocation of optical detectors. Further, the converted electrical signal may need to pass a plurality of metallization layers before they reach the logic gates. Therefore, the cost and manufacturing complexity increase. If, however, the optical paths are placed on the back of the substrates, since silicon substrates are substantially opaque to most of the commonly used wavelengths, the wavelength of the light needs to be increased in order to penetrate through silicon substrates. This not only applies extra requirements to the optical source, the strength of light received by the photon detectors will also be attenuated, and hence the optical detectors need to be more sensitive.
Accordingly, what is needed in the art is an integrated circuit structure that may incorporate photon detectors to take advantage of the benefits associated with reduced power consumption while at the same time overcoming the deficiencies of the prior art. 